The prior art shows a number of three terminal, transistor-like devices formed from layered structures. Some require the use of metals operating in a superconducting mode while others require the use of normal metals or combinations of such metals. In addition, some of these structures call for the use of tunnel barriers, insulators or crystalline materials which separate the metals one from the other. To the extent that such prior art arrangements have not come into wide use, it is apparent that they are lacking in certain qualities which are present in known semiconductor transistor devices. Apart from this, while three terminal, cryogenic devices have appeared in the prior art, for reasons like poor gain and lack of isolation, none of these devices have filled the need for a transistor-like device operating in the cryogenic environment.
U.S. Pat. No. 3,155,886 filed Aug. 16, 1961 shows three superconductors separated by a pair of tunnel barriers. In this instance, the tunnel barrier between the second and third superconductor is a high band gap material.
U.S. Pat. No. 3,204,115 filed July 31, 1961 shows an arrangement similar to the '886 reference. One difference is that the first and third materials are normal metals.
U.S. Pat. No. 3,275,844 filed Nov. 16, 1962 discloses a transistor-like structure similar to the '886 reference except that all metals are normal metals.
U.S. Pat. No. 3,372,315 filed Aug. 4, 1965 shows a device similar to the '844 reference except that the second insulator is a single crystal material (mica) and has a "low" energy barrier height. This device was intended to run at room temperature. While the reference calls for a low barrier, in reality, the barrier height is rather high (0.8-0.95 eV) relative to that required (.about.0.005 eV) for the semiconductor of the present application. In addition, the reference has no requirement for a superconducting base.
U.S. Pat. No. 3,178,594 filed June 27, 1962 shows a transistor-like device wherein the middle metal film is required to be superconducting whereas the other two may be normal or superconducting. The insulators between films are, however, both high energy gap materials and of a thickness which permits tunnelling.
U.S. Pat. No. 4,157,555 filed Nov. 7, 1977 shows a superconducting transistor composed of three thin films of superconducting material separated from each other by insulating oxides of a thickness sufficient to permit tunnelling. One junction is biased above twice the superconducting energy gap and the other is biased at less than twice the superconducting energy gap. Injection of quasiparticles into the center film by one junction provides a current gain in the second junction. The semiconductor layer of the present application improves on this reference by blocking the return flow of quasiparticles thereby keeping the common base current gain equal to or less than one. Also, in the present application, there is no tunnelling between the base and collector regions. By providing a low energy barrier material, the transmission of quasiparticles is encouraged while the passage of Cooper pairs is inhibited.
U.S. Pat. No. 4,334,158 filed June 6, 1980 shows a three terminal device called a QUITERON in which the superconducting gap of a superconducting material is caused to vanish by the injection of excess quasiparticles. The device uses a pair of tunnel barriers and specifically shows that the tunnel barrier of the injector junction may be a semiconductor. It should be appreciated that in this reference, the semiconductor has no other function than its use as a tunnel barrier. To the extent that the device of this reference suffers from poor input/output isolation, the device of the present application, in addition to having gain, has excellent input/output isolation.
It is, therefore, an object of the present invention to provide a three terminal, transistor-like device which exhibits gain and has excellent input/output isolation.
Another object is to provide a three terminal, transistor-like device which exhibits good linearity and has low power dissipation.
Still another object is to provide a three terminal, transistor-like device which exhibits high speed switching at cryogenic temperatures. Yet another object is to provide a three terminal, transistor-like device which can be formed into high density integrated circuits.